16
responsible for the difference between the expected (0.7 °C) and observed (0.9 °C)
rise in GMST over the course of the Anthropocene. Most importantly, the calcula-
tion conducted above reveals quantitative consistency between the observed and
expected rise in global temperature over the course of the Anthropocene. This is a
critically important first step in the attribution of global warming to humans.
Several other aspects of global warming bear the human fingerprint. Climate mod-
els predict that as GHGs rise, the lowest layer of the atmosphere (the troposphere) will
warm while the second layer of the atmosphere (the stratosphere) should cool.
Stratospheric cooling is a consequence of the blanketing effect of GHGs: as atmo-
spheric levels of GHGs rise, a larger fraction of the thermal energy emitted by the
surface is absorbed, re-emitted, and therefore blocked from reaching the stratosphere.
As shown in Fig. 1.5, tropospheric warming coupled with stratospheric cooling is seen
in the climate record, at least over the part of the Anthropocene for which modern
measurements of atmospheric temperature profiles exist (Sherwood and Nishant 2015 ).
About two-thirds of the cooling of the upper stratosphere for the time period 1979–
2005 has been attributed to rising GHGs, with the remainder attributed to human-
induced depletion of stratospheric O 3 (Mitchell 2016 ). The pattern of temperature
changes with respect to altitude and latitude throughout the troposphere and strato-
sphere agrees with the pattern predicted by climate models to a high degree of statisti-
cal significance (Santer et al. 2013a), although these models do tend to overestimate the
amount of warming observed in the lower atmosphere (Santer et al. 2013b).^10
Another important human fingerprint of global warming is the observation that
the altitude of the tropopause, the boundary between the troposphere and the strato-
sphere, has been rising as predicted by climate models (Santer et al. 2013a). Had
modern global warming been due to an increase in the luminosity of the Sun or a
release of energy from the world’s oceans, scientists would have expected to observe
warming in the stratosphere and troposphere as well as little to no change in the
height of the tropopause.
1.2.3.2 Carbon Dioxide
Carbon dioxide (CO 2 ) is the single greatest waste product of modern society. There
is compelling scientific evidence that the rise in atmospheric CO 2 during the
Anthropocene is due, nearly entirely, to human activity. The rise in CO 2 from 1765
to the early 1900s was predominately driven by the clearing of forests for agricul-
ture (also known as land use change, or LUC) (Siegenthaler and Oeschger 1987 ).
For a few decades subsequent to 1900, LUC and the combustion of fossil fuels
made nearly equal contributions to the rise in atmospheric CO 2. Since the early
1950s, the growth of atmospheric CO 2 has been driven primarily by the combustion
of fossil fuels^11 (Le Quéré et al. 2015 ).
(^10) The tendency of many climate models to overestimate observed warming is a central theme of
Chap. 2.
(^11) Combustion of fossil fuels refers to the burning of coal, oil and gasoline, as well as natural gas
(mainly methane) to meet society’s needs for heat, electricity, transportation, and various other
industrial enterprises.
1 Earth’s Climate System